1. Field of the Invention
This invention generally relates to the technology of forming color images excellent in terms of coloring property and color evenness. More particularly, the present invention relates to a liquid composition optimally applicable to image forming processes using an ink-jet recording method as well as to an ink set, an image forming method and an image forming apparatus using such a liquid composition.
2. Related Background Art
With the ink-jet recording method, ink is made to fly and stick to a recording medium such as a sheet of paper for recording. Japanese Patent Publications Nos. 61-59911, 61-59912 and 61-59914 disclose an ink-jet recording method using an electrothermal transducer as ejection energy supply means to provide ink with thermal energy and generate a bubble there so as to make it possible to eject liquid droplets. The disclosed method provides an advantage that it allows the use of a high density multi-orifice recording head so that a high resolution and high definition image can be recorded at high speed.
Meanwhile, ink to be used with the known ink-jet recording method contains water as principal ingredient along with a water-soluble high boiling point solvent such as glycol added thereto for the purpose of preventing ink from drying in the nozzle and clogging the latter. Such ink is short of being positionally fixed satisfactorily on the recording medium and can produce an uneven image probably because of the uneven distribution of the filler and the sizing agent on the surface of the recording medium, which is typically a sheet of paper. On the other hand, there has arisen a strong demand for high quality ink-jet recording comparable to the quality level of silver salt photography. Thus, currently there is a strong technological demand for raising the optical density of images formed by ink-jet recording, broadening the spectrum of color reproducibility of ink-jet recording and improving the color evenness of recorded images.
Under these circumstances, various techniques have been proposed to enhance the stability of ink-jet recording and improve the quality of images formed by ink-jet recording. According to certain techniques proposed for recording medium, a filler and a sizing agent are applied to the surface of the base paper of recording medium. For example, porous micro-particles adapted to adsorb coloring materials are applied to the base paper to form an ink receiving layer of porous micro-particles on it. Coat paper (certain types of coated paper) for ink-jet recording is currently marketed as recording medium realized on the basis of such techniques.
As for techniques concerning recording liquid that is to be ejected on the recording medium in the form of a jet stream, there is one proposed for applying ink and a processing solution chemically sensitive to ink onto the recording medium in such a way that they react with each other on the recording medium. Ink-jet printers that utilizes this technique have also been marketed.
For instance, Japanese Patent Application Laid-Open No. 63-60783 discloses a recording method of causing a liquid composition containing a basic polymer to adhere to a recording medium and subsequently recording an image by using inks containing an anionic dye and Japanese Patent Application Laid-Open No. 63-22681 discloses a recording method of mixing a first liquid composition containing reactive chemical species and a second liquid composition containing compounds that react with the reactive chemical species on a recording medium. Furthermore, Japanese Patent Application Laid-Open No. 63-299971 discloses a recording method of applying a liquid composition containing an organic compound having two or more than two cationic radicals per molecule onto a recording medium and subsequently recording an image by using ink containing an anionic dye and Japanese Patent Application Laid-Open No. 64-9279 discloses a recording method of applying an acidic liquid composition containing succinic acid and subsequently recording an image by using ink containing an anionic dye. Japanese Patent Application Laid-Open No. 64-63185 discloses a recording method of applying a liquid composition that makes dyes insoluble before recording an image by using ink.
Additionally, Japanese Patent Application Laid-Open No. 8-224955 discloses a recording method of using a liquid composition containing a first cationic substance showing a different molecular weight distribution range and ink containing an anionic coloring material and Japanese Patent Application Laid-Open No. 8-72393 discloses a liquid composition containing a cationic substrate and finely divided cellulose with ink. These patent documents suggest that they can produce excellent images in terms of high optical density, printing quality and water-resistance as well as color reproducibility and bleeding. Japanese Patent Application Laid-Open No. 55-150396 proposes a recording method of recording an image on a recording medium by means of a dye ink and subsequently applying a water-proofing agent adapted to form a lake there as means for making the recorded image water-resistant.
On the other hand, there has been recognized a problem called bleeding, which is a phenomenon occurring when a plurality of inks of different colors are applied adjacent to each other onto a recording medium for forming a multi-color image and can blur the image by the mixing of inks with each other at the boundary areas of the image.
To avoid the problem, proposals have been made for accelerating the fixing process of ink on the recording medium (such as recording paper). Japanese Patent Application Laid-Open No. 55-65269 discloses a method of adding a compound for improving the permeability of ink such as a surface active agent to ink and Japanese Patent Application Laid-Open No. 55-66976 discloses the use of ink containing a volatile solvent as principal ingredient. However, while the former method of adding a surface active agent to ink may improve the permeability of ink relative to the recording medium to make the ink advantageous in terms of fixation or bleeding, the coloring materials in the ink can also permeate deep into the recording medium to disadvantageously reduce the optical density and the saturation of the image formed on the recording medium.
The recording medium obtained by applying a filler and a sizing agent to the surface of base paper (hereinafter referred to as coated paper) has been recognized as a technological achievement for forming high quality images.
It is known that the coloring material applied to the surface of a recording medium generally has to be made to remain in an isolated molecular state without being coagulated on the surface in order to obtain a highly saturated image. The porous micro-particles of coat paper operates to show such a functional feature. However, in order to obtain a satisfactory level of optical density and saturation for the image formed thereon, a thick ink receiving layer has to be formed on the surface of base paper by applying porous micro-particles at an enhanced rate to completely cover the base paper so that, as a result, the texture of the base paper can be lost. To begin with, the inventors of the present invention presumed that such a thick ink receiving layer is needed at the cost of the texture of the base paper because the coloring material is not adsorbed by the porous micro-particles effectively and efficiently.
The following description is based on an assumption of using coat paper having a single ink receiving layer. FIG. 9 of the accompanying drawings is a schematic cross sectional view of a type of coat paper, showing only a part of the surface thereof. Referring to FIG. 9, there are shown base paper 901 and an ink receiving layer 903. Generally, the ink receiving layer contains porous micro-particles 905 and an adhesive agent 907 for rigidly holding them. As ink is applied to the surface, it permeates to fill the gaps among the porous micro-particles 905 due to the capillarity of the gaps to form permeating ink sections 909. Since the ink receiving layer shows a locally varying density of porous micro-particles as shown in FIG. 9, ink may permeate in various different ways although it does due to the capillarity of the gaps. Therefore, the coloring material contained in the ink may not be able to contact evenly to the surfaces of the porous micro-particles so that they may not be adsorbed effectively and efficiently by the porous micro-particles.
Additionally, there may arise areas where ink is prevented from permeating by the adhesive agent 907 so that there may be areas in the ink receiving layer 903 where ink cannot get into. Such areas do not operate at all for coloring. For the above reasons, porous micro-particles cannot effectively and efficiently adsorb coloring materials in an isolated molecular state on conventional coat paper so that they have to be applied at an enhanced rate at the cost of the texture of the coat paper.
On the basis of the above novel finding, the inventors of the present invention came to find that it is possible to cause coloring materials and micro-particles to react each other in a liquid-liquid state by using micro-particles adapted to adsorb the coloring materials and dispersing the micro-particles in a liquid phase so that both the coloring materials and the micro-particles may be present in a liquid state, whereby images improved in optical density and saturation can be formed.
In view of the above described technological background, the inventors of the present invention came to select the following objectives.
(1) Since it is important to make the given ink and coloring material operate more effectively and efficiently for the improvement of the optical density and the saturation. Therefore, it is an object of the present invention to provide an ink set that can form an image with a high optical density and a high degree of saturation by ink-jet recording.
(2) It is also an object of the present invention to provide an ink set that can alleviate the problem of bleeding appearing along the boundary areas of a multi-color image formed by applying a plurality of inks of different colors where different colors of the image contact with each other.
It is still another object of the present invention to provide an ink-jet recording method that can form a high quality multi-color image showing a high degree of optical density and saturation on a recording medium by means of ink-jet without damaging the texture of the recording medium and without being accompanied by the problem of recognizable bleeding appearing along the boundary areas of the multi-color image.
Still another object of the present invention is to provide an ink-jet recording apparatus that can form a high quality multi-color image on a recording medium by means of ink-jet without being accompanied by the problem of recognizable bleeding appearing along the boundary areas of the multi-color image and also a recording unit and an ink cartridge to be used with such an ink-jet recording apparatus. A further object of the present invention is to provide a method of alleviating the problem of bleeding of an image showing a high degree of optical density and saturation and formed by means of an ink-jet recording method.
In an aspect of the present invention, the above objects are achieved by providing a liquid composition containing a first cationic substance and a second cationic substance different from the first cationic substance, the first cationic substance being in the form of micro-particles, the micro-particles being cationic at least at the surface thereof.
In another aspect of the present invention, there is provided a liquid composition to be used with an anionic aqueous ink containing a coloring material so as to react with the ink on a recording medium for printing on the recording medium, the liquid composition containing a first cationic substance and a second cationic substance different from the first cationic substance, the first cationic substance being in the form of micro-particles, the micro-particles being cationic at least at the surface thereof.
In still another aspect of the present invention, there is provided an ink set comprising in combination a liquid composition according to the invention and at least an anionic ink selected from a group of inks of yellow, magenta, cyan, black, red, blue and green.
In still another aspect of the present invention, there is provided an image forming method comprising a step of applying a liquid composition according to the invention and an ink containing an anionic coloring material to a recording medium so as to make them contact with each other on the recording medium.
In still another aspect of the present invention, there is provided an image recording apparatus provided with a recording unit comprising an ink storage section for storing separately a liquid composition according to the invention and an ink of at least one color containing an anionic substance and a head section for separately ejecting the liquid composition and the ink as liquid droplets.
In still another aspect of the present invention, there is provided a liquid composition adapted to form colored sections on a recording medium when applied to the recording medium with an ink containing an anionic or cationic coloring material:
the liquid composition containing a first compound showing the polarity opposite to that of the ink and a second compound also showing the polarity opposite to that of the ink;
the first compound being in the form of micro-particles;
the coloring material in the ink being adapted to be adsorbed or bound to the surface of the micro-particles while its molecular state being substantially equivalent to that in the ink at the time of mixing the liquid composition and the ink in a liquid state so as to make the dispersion of the micro-particles unstable and the micro-particles coagulate with each other.
In still another aspect of the present invention, there is provided an ink set comprising as independent components thereof an ink containing an anionic or cationic coloring material and a liquid composition containing micro-particles electrically charged to the polarity opposite to that of the coloring material;
the liquid composition containing a first compound showing the polarity opposite to that of the ink and a second compound also showing the polarity opposite to that of the ink;
the coloring material in the ink being adapted to be adsorbed or bound to the surface of the micro-particles while its molecular state being substantially equivalent to that in the ink at the time of mixing the liquid composition and the ink in a liquid state so as to make the dispersion of the micro-particles unstable and the micro-particles coagulate with each other.
In still another aspect of the present invention, there is provided an image forming method comprising a step of causing an ink containing a coloring material and a liquid composition containing a first compound in the form of micro-particles reactive to the coloring material and a second compound also reactive to the coloring material to react with each other as liquid-liquid reaction on a recording medium and forming colored sections containing the coloring material and the first and second compounds;
the step including a step of causing the coloring material in the ink to be adsorbed or bound to the surface of the micro-particles in the liquid composition, while maintaining the molecular state of the coloring material in the ink.
In still another aspect of the invention, there is provided an image forming method comprising a step of causing an ink containing a coloring material and a liquid composition containing a first compound in the form of micro-particles reactive to the coloring material and a second compound also reactive to the coloring material to react with each other as liquid-liquid reaction on a recording medium and forming colored sections containing the coloring material and the first and second compounds;
the step including:
a step of causing the coloring material in the ink to be adsorbed or bound to the surface of the micro-particles in the liquid composition, while maintaining the molecular state of the coloring material in the ink; and
a step of causing the coloring material in the ink to react with the second compound in the liquid composition.
In still another aspect of the invention, there is provided an image forming method comprising a step of causing an ink containing a coloring material and a liquid composition containing a first compound in the form of micro-particles reactive to the coloring material and a second compound also reactive to the coloring material to react with each other as liquid-liquid reaction on a recording medium and forming colored sections containing the coloring material and the first and second compounds;
the step including:
a step of causing the coloring material in the ink to be adsorbed or bound to the surface of the micro-particles in the liquid composition, while maintaining the molecular state of the coloring material in the ink;
a step of causing the micro-particles to become less dispersed and coagulate as a result of adsorbing or binding the coloring material to the surface thereof; and
a step of causing the coloring material in the ink to react with the second compound in the liquid composition.
In still another aspect of the invention, there is provided an image forming method comprising a step of causing an ink containing a coloring material and a liquid composition containing a first compound in the form of micro-particles reactive to the coloring material and a second compound also reactive to the coloring material to react with each other as liquid-liquid reaction on a recording medium and forming colored sections containing the coloring material and the first and second compounds;
the step including:
a step of causing the coloring material in the ink to be adsorbed or bound to the surface of the micro-particles in the liquid composition, while maintaining the molecular state of the coloring material in the ink;
a step of causing the micro-particles to become less dispersed and coagulate as a result of adsorbing or binding the coloring material to the surface thereof; and
a step of causing the part of the coloring material not participating in the reaction with the micro-particle in the liquid composition to react with the second compound in the liquid composition.
In still another aspect of the present invention, there is provided an image formed by causing an ink containing a coloring material and a liquid composition containing a first compound in the form of micro-particles reactive to the coloring material and a second compound also reactive to the coloring material to react with each other as liquid-liquid reaction on a recording medium, the image containing colored sections therein;
the colored sections containing the coloring material of the ink in a state of being adsorbed or bound to the micro-particles of the liquid composition as single molecules and in a state of having reacted with the second compound of the liquid composition.
In a further aspect of the present invention, there is provided a method of alleviating bleeding appearing along the boundary lines of first areas of a color and second areas of another color located respectively adjacent to the first areas in an image formed by applying liquid droplets to a recording medium;
at least either the first areas or second areas being colored sections formed by causing ink containing a coloring material and a liquid composition containing a first compound in the form of micro-particles reactive to the coloring material and a second compound also reactive to the coloring material to react with each other as liquid-liquid reaction on a recording medium, the colored sections containing therein the coloring material of the ink in a state of being adsorbed or bound to the micro-particles of the liquid composition as single molecules and in a state of having reacted with the second compound of the liquid composition.
While it is not still clear why the objects of the present invention are achieved by the above described arrangements, the theories proposed by the inventors of the present invention will be briefly discussed below.
One of the theorems underlying the fact that an image showing a highly coloring property is obtained according to the invention is that the coloring material contained in the ink is dispersed and positionally fixed without becoming laked by utilizing the electrostatic force that arises between the molecules of the coloring material and the micro-particles. On the other hand, the ink becomes less bleeding and water-resistant as a result of allowing the coloring material to become laked. While this may appear contradictory, it seems that both the coloring effect and the effect of becoming less bleeding and water-resistant of the coloring material are made to coexist in a well-balanced subtle way. To prove this, the inventors of the present invention conducted an experiment, which will be discussed below. Firstly, a liquid composition same as the one described above in terms of any of the different aspects of the invention except that the second cationic substance was not contained was prepared and mixed with ink to observe the ratio of the dye molecules adsorbed to the micro-particles. While the ratio varied slightly depending on the type of the micro-particles and the type and composition of the ink, it was found that about ⅓ of the overall dye molecules contained in the ink were adsorbed to the micro-particles. In other words, a highly coloring property can be obtained satisfactorily for ink when ⅓ of the overall dye molecules contained in the ink are dispersed in the surface layer of a sheet of paper in an isolated state. Then, the remaining ⅔ of the dye molecules that are not constrained by the cationic micro-particles are fixed deep in the paper as the ink permeates there, although ink can flow into adjacent sections of the image to give rise to a bleeding phenomenon if the paper is plain paper where ink permeates only slowly. While the stripy unevenness of the formed image may be dissolved by the bleeding of ink if ink flows into adjacent sections of the image only moderately, it is essential to control the flow of ink and hence that of the part of the coloring material contained in the ink that does not participates in the reaction with the cationic micro-particles in order to reliably alleviate the bleeding. Additionally, some of the molecules of the dye that are not positionally constrained can also flow out when the image formed on the sheet of paper is wetted if partly. Still additionally, it was found some of the micro-particles can also flow out, carrying the dye molecules adsorbed thereto, under certain conditions because they are not positionally constrained either. Thus, these facts explains why the effect of becoming less bleeding and water-resistant is not remarkable on certain plain paper.
Based on the above results of the experiment, the inventors of the present invention came to believe that both the highly coloring effect and the effect of alleviating the bleeding and making the coloring material water-resistant can be realized simultaneously when some of the molecules of the coloring material are adsorbed to the micro-particles and positionally fixed as independent molecules in the surface layer of the recording paper and the remaining molecules of the coloring material that are not adsorbed to the micro-particles are turned insoluble by the cationic compound and positionally constrained.
As a matter of fact, the coloring property and the effect of suppressing the bleeding phenomenon and becoming water-resistant of the ink were improved in the above experiment when the liquid composition contained a first cationic compound in a state of micro-particles and a second cationic compound in a well balanced way.
If compared with a cationic polymer showing a high molecular weight, a water-soluble metal salt that is used as cationic compound with micro-particles for the purpose of the present invention makes the coloring material less coagulating and more positionally constrained to improve the coloring property, the water resistance and the effect of suppressing the bleeding.
It was also found that the use of a water-soluble cationic metal salt provides additional advantages of improving the preservability of the liquid composition particularly in terms of the effect of suppressing the thixotropic property due to a raised viscosity in a low temperature environment. While the reason for this is not clear, the inventors of the present invention presume that the water-soluble metal salt in the liquid composition affects the surface condition of the micro-particles in some way or another to raise the electrostatic repelling force and consequently improve the dispersibility of the micro-particles at low temperatures. If the water-soluble cationic metal salt becomes acidic in water, it is possible to eliminate the need of using acid or suppress the rate of adding it in order to accelerate the ionization of the surface of the micro-particles and encouraging the micro-particles to disperse quickly. Therefore, the use of such a salt is preferable from the viewpoint of corrosion resistance and safety.
As a result of a series of experiments conducted by the inventors of the present invention, it was found that the total amount of the water-soluble cationic metal salt is preferably less than that of the micro-particles in the liquid composition. More preferably, the ratio of the total amount of the water-soluble cationic metal salt to that of the micro-particles in the liquid composition is ⅕ or less (in terms of weight).